I have a row of Little Gem Magnolias planted below a retaining wall that is about 20 feet high. The trees are now growing above the wall and blocking the view. Is it okay to top them so that they don't grow any higher?

speaking generally as I'm not familiar with how this particular tree would react, topping of trees is not recommended, it is better to use a method called drop-crotching. This article explains both, although the diagrams of drop-crotching aren't especially clear and you could probably find better ones.